CO2 incorporation in hydroxide and hydroperoxide containing water clusters—a unifying mechanism for hydrolysis and protolysis

Abstract

The reactions of CO₂ with anionic water clusters containing hydroxide, OH⁻(H₂O)_n, and hydroperoxide, HO₂⁻(H₂O)_n, have been studied in the isolated state using a mass spectrometric technique. The OH⁻(H₂O)_n clusters were found to react faster for n = 2,3, while for n >3 the HO₂⁻(H₂O)_n clusters are more reactive. Insights from quantum chemical calculations revealed a common mechanism in which the decisive bicarbonate-forming step starts from a pre-reaction complex where OH⁻ and CO₂ are separated by one water molecule. Proton transfer from the water molecule to OH⁻ then effectively moves the hydroxide ion motif next to the CO₂ molecule. A new covalent bond is formed between CO₂ and the emerging OH⁻ in concert with the proton transfer. For larger clusters, successive proton transfers from H₂O molecules to neighbouring OH⁻ are required to effectively bring about the formation of the pre-reaction complex, upon which bicarbonate formation is accomplished according to the concerted mechanism. In this manner, a general mechanism is suggested, also applicable to bulk water and thereby to CO₂ uptake in oceans. Furthermore, this mechanism avoids the intermediate H₂CO₃ by combining the CO₂ hydrolysis step and the protolysis step into one. The general mechanistic picture is consistent with low enthalpy barriers and that the limiting factors are largely of entropic nature.